The present invention relates to a device and method for measurement of incident power and energy and, more particularly, to devices and methods which reduce errors in measurement due to variations in background radiation.
The term “power” as used herein refers to any measure of applied energy expressed as a function of time (e.g. 1 Watt=1 joule/s).
Previously available thermal power meters based on the thermopile effect are used to measure the powers and energies of lasers or other light sources. These meters are based on the effect of heat flow through the sensor causing a temperature difference, which is converted by a thermopile into a voltage proportional to the power flowing through the device. FIG. 1 shows such a previously available device 20 based on use of a commercial thermoelectric cooler (TEC; 24) used in a reverse fashion where instead of a current flow through the device causing a temperature difference across the faces, flow of heat 22 induced by incident energy 26 causes a temperature difference across the faces which, in turn causes a current flow through a resistor and hence a voltage essentially proportional to the power applied to the device. A pulse of incident energy 26 (e.g. from a laser beam) applied to an absorbing surface 28 can also be measured by integrating the voltage produced over time where the energy is ∝ ∫ power dt (FIG. 2). Devices of this type are used typically for powers from 1 mW to 10 Watts and energies of 0.1 mJ to 10 J. Devices of this type rely upon a heat sink disc 30 (FIG. 1) to minimize the effect of ambient energy from other sources.
Devices of this type are inherently limited with respect to measurement of low powers and energies due to interference from ambient background radiation and temperature. Since devices of this type measure a total temperature difference induced on the sensor from any source, the accuracy of the measurement is adversely affected because the light source being measured, changes in air temperature, and changes due to uneven radiation on the case of the measuring device all contribute to the total temperature difference. For instance, with the Ophir model 3A-P (Ophir Optronics, Jerusalem, Israel), although the power noise level of the device is rated at 10 μW, the long term change in reading of power over a measurement period of approximately 1 hour under typical room conditions is more than 300 μW even though the meter is enclosed in an insulating housing. Thus devices of this type introduce an unacceptable level of error into measurements, especially when measuring applied powers less than ˜1 mW.
U.S. Pat. No. 5,376,783 attempts to address the problem of background radiation as it affects measurement. However, this patent has, as an inherent limitation, a strict requirement for the measurement of light.
There is thus a widely recognized need for, and it would be highly advantageous to have, a device and method for measurement of incident power and energy devoid of the above limitation.